The present invention relates to a structure of a lift gate for a vehicle which is provided to open or close an opening portion formed at a rear portion of the vehicle, in particular, which comprises an inner panel and an outer panel which are made from synthetic resin.
Conventionally, a lift gate provided to open or close an opening portion formed at a rear portion of a hatch back type of passenger vehicle or the like comprises an inner panel and an outer panel which are made of a steel plate and joined to each other at their respective outer edge end portions. Herein, since the weight of a vehicle body influences the fuel consumption (fuel economy) of the vehicle considerably, developments of making the lift gate from synthetic resin have been actively conducted for reduction of the vehicle-body weight. The synthetic-resin made inner and outer panels can make the vehicle-body weight more lightweight than the steel-plate made inner and outer panels. Further, the synthetic-resin made outer panel enables integral forming of exterior components, such as a spoiler, and the synthetic-resin made inner panel enables integral forming of interior components, such as a trim. Accordingly, the number of vehicle components or manufacturing costs can be reduced properly.
The lift gate is a relatively large-sized member constituting the vehicle body compared with other members, such as a side door or a trunk lid, and rotates greatly around a hinge fixed to a rear header when it opens or closes. Accordingly, the lift gate is required to be made to have a high rigidity against a bending stress and a torsional stress which may be caused by a lift gate's opening or closing operation. Therefore, in making the lift gate from synthetic resin, it has been tried to additionally attach plural reinforcements to specific portions where the bending and torsional stresses concentrate, thereby increase the rigidity of the lift gate properly.
Japanese Patent Laid-Open Publication No. 2009-67360 discloses the lift gate structure for a vehicle, in which the synthetic-resin made outer panel comprises the upper portion including the transparent window portion and the spoiler portion and the lower portion equipped with the license-plate attachment portion, and this outer panel and the synthetic-resin made inner panel are joined to each other at their respective outer edge end portions via the adhesive. Thereby, the window portion can be made from synthetic resin in place of a window glass (windowpane), so that an attachment structure for the window portion can be made simpler and the weight of the lift gate can be reduced.
The lift gate structure disclosed in the above-described patent document comprises the synthetic-resin made rear window which is integrally formed at the outer panel, so that the simple lift-gate structure and the lightweight lift gate can be provided. However, the window opening portion of the lift gate may be designed to have a relatively wide area from perspectives of passenger's comfortableness, better design and the like. Herein, in a case in which this wide window opening portion is made from transparent synthetic resin in place of the window glass (windowpane), since the synthetic resin itself is lighter than the glass, there is a concern that the sufficiently-large rigidity required as the rear window or appropriate accuracy of components may not be obtained. Meanwhile, in a case in which the glass-made rear window (rear windowpane) is used and attached to the synthetic-resin made lift gate, the weight of the rear window glass (rear windowpane) acts on the lift gate in addition to the bending and torsional stresses caused by the lift gate's opening or closing operation, so that the rigidity large enough to properly support these weight and stresses should be required. Accordingly, it may be necessary that the plate thickness of the inner panel and the outer panel is made thicker or some reinforcement is provided additionally.
On the other hand, the outer surface of the outer panel forms a decorative face as an exterior design, so it generally has a relatively smooth and wide face portion compared to the inner panel to which some vehicle equipments are installed. This face portion is likely to generate surface vibrations caused from vehicle vibrations or a dent caused by pressing during the lift gate's opening or closing operation. These surface vibrations or dent may influence the product values of the lift gate improperly. In particular, in a case in which the outer panel is made from synthetic resin, since generations of the surface vibrations or the dent become greater compared to the case of the steel-plate made outer panel, the product values of the lift gate may further deteriorate. Therefore, it may become necessary in order to properly increase the face rigidity of the outer panel that the plate thickness of the outer panel is made thicker or some reinforcing ribs or reinforcements are provided at a back face of the face portion additionally.
However, the increase of the plate thickness or the addition of the reinforcements and the like for increasing the rigidity of the lift gate may cause the weight increase of the lift gate as described above. That is, despite aiming at the lightweight lift gate by making the inner and outer panels from synthetic resin, the weight increase of the lift gate may be caused by the thicker panels or the additional provision of the reinforcements and the like for solving lack of the rigidity. Consequently, there is a concern that the effect of weight reduction by the synthetic-resin made lift gate may improperly decrease.